The present invention relates to an image forming apparatus such as a printer, a facsimile machine, and a copying machine that forms a toner image using electrophotography.
Japanese Patent Publication No. 2003-195712A discloses an image forming apparatus that comprises:
an image carrier on which a toner image is formed;
an intermediate transfer member for forming a primary transfer position contacting the image carrier so as to perform primary transfer of the toner image on the image carrier, and then forming a secondary transfer position contacting a recording medium so as to transfer the toner image onto the recording medium;
a secondary transfer roller brought into press contact with the intermediate transfer member via the recording medium at the secondary transfer position;
a gate roller pair arranged in an immediate vicinity of a preceding stage of the secondary transfer position relative to a direction of conveyance of the recording medium, and thereby conveying the recording medium toward the secondary transfer position;
a cleaning blade retractably brought into contact with the intermediate transfer member to scrape off residual toner remaining on the intermediate transfer member after the secondary transfer; and
a fuser for allowing the recording medium onto which the toner image has been transferred at the secondary transfer position to pass therethrough, and thereby fusing the toner image onto the recording medium.
In such an image forming apparatus, if the cleaning blade would be brought into contact with the intermediate transfer member during the above-mentioned primary transfer, a vibration in the apparatus arising at the time of contact of the cleaning blade or a change in the moving load of the intermediate transfer member could easily cause deviation or distortion in the obtained image.
In order to resolve such a problem, in the image forming apparatus, regardless of the thickness of the recording medium, it is configured that the cleaning blade is brought into contact with the intermediate transfer member in a time duration that primary transfer is not performed.
In general, in an image forming apparatus as described above, the conveyance speed of the recording medium caused by the gate roller pair is preferably set to be faster than the conveyance speed of the recording medium at the secondary transfer position. This is because if the conveyance speed of the recording medium caused by the gate roller pair in the immediate vicinity of the preceding stage of the secondary transfer position were equal to or slower than the conveyance speed of the recording medium at the secondary transfer position, a tension could act on the recording medium when the recording medium is located between the gate roller pair and the secondary transfer position described above. Then, this tension could serve as a load at the secondary transfer position, and thereby could cause instability in the transfer operation at the secondary transfer position (deviation or distortion could arise in the transferred image).
In contrast, when, the conveyance speed of the recording medium caused by the gate roller pair is set to be faster than the conveyance speed of the recording medium at the secondary transfer position, the above-mentioned tension does not act. Thus, when the recording medium has a thickness at or below that of an ordinary sheet (a sheet stiffness), no problem arises from the above-mentioned tension.
Nevertheless, in the case that the recording medium conveyed by the gate roller pair and the secondary transfer position described above is a thick sheet, a compressive force acts on the thick sheet located between the gate roller pair and the secondary transfer position. This compressive force acts such as to increase the traveling speed of the intermediate transfer member at the secondary transfer position, and hence tends to cause instability in the traveling speed of the intermediate transfer member.
Thus, in such a situation, when the cleaning blade is brought into contact with the intermediate transfer member, the traveling speed of the intermediate transfer member becomes more unstable so that the speed fluctuation increases. This easily causes deviation or distortion in the image under the primary transfer in the above-mentioned primary transfer position.
Irrespective of the thickness of the recording medium, similar problem would arise in the case of a recording medium having a large stiffness that is enough to cause an increase in the traveling speed of the intermediate transfer member at the secondary transfer position and thereby causes an instability in the traveling speed of the intermediate transfer member. Thus, in the present application, a “thick sheet” indicates a “recording medium having a large sheet stiffness that is enough to cause an increase in the traveling speed of the intermediate transfer member at the secondary transfer position and thereby causes an instability in the traveling speed of the intermediate transfer member.”
Nevertheless, in order that the cleaning blade is bought into contact with the intermediate transfer member in a time duration that primary transfer is not performed, in other words, in order that primary transfer is not performed when the cleaning blade comes in contact with the intermediate transfer member, spacing need be increased between the formed images. Then, if the spacing were increased between the formed images, this would reduce the achievable number of achievable image formations per unit time.
In the apparatus described above, since the cleaning blade is brought into contact with the intermediate transfer member in a time duration that primary transfer is not performed, regardless of the thickness of the recording medium, the achievable number of achievable image formations per unit time is reduced for the recording medium of every thickness.